According to a known process for producing reduced iron, fine ore or lump ore is reduced in the solid phase in a counter-flow shaft furnace using a reducing gas prepared by reforming natural gas to produce reduced iron. This process, however, requires a large supply of natural gas, which is expensive as a reducing agent, and generally has limitations such as plant siting limited to regions where natural gas is produced.
Accordingly, processes for producing reduced iron using coal as a reducing agent, instead of natural gas, have recently attracted attention. Coal is relatively less expensive and eases geographical limitations on plant siting. Such processes for producing reduced iron using coal as a reducing agent are exemplified by a known process described below. A raw material containing a metal oxide such as iron oxide is mixed with a carbonaceous material. The mixture is then dried and agglomerated under such conditions as to generate volatile matter. For the volatile matter to function as a binder, the dried mixture is heated and compressed to prepare green compacts. The green compacts are charged into a rotary hearth furnace and are reduced by heating at 2,150° F. to 2,350° F. (1,177° C. to 1,288° C.) for 5 to 12 minutes to produce reduced iron.
According to this process, if the content of the volatile matter, which functions as a binder, in the coal is less than 20% by mass, the green compacts require an additional organic binder. If the content of the volatile matter is 20% to 30% by mass, the green compacts require compression above 10,000 lb/in2 (703 kg/cm2) and heating at 800° F. (427° C.). If the content of the volatile matter exceeds 30% by mass, the green compacts only require compression above 10,000 lb/in2 (703 kg/cm2). The carbonaceous material used is preferably a coal having a high fixed carbon content and a volatile matter content of about 20% by mass or more, such as bituminous coal.
If the reduced iron discharged from the rotary hearth furnace has an excess carbon content of 2% to 10% by mass, the excess carbon advantageously increases the rate of reduction to promote complete reduction. In addition, the excess carbon may be utilized as carbon for steelmaking in an electric furnace.
Because the green compacts (hereinafter also referred to as agglomerates with the carbonaceous material incorporated therein) are porous, they have insufficient contact between the carbonaceous material and the metal oxide, such as iron ore, and thus exhibit low thermal conductivity and a low reduction rate. A process has been attempted in which a carbonaceous material that exhibits lower maximum fluidity in softening melting is used for the agglomerates with the carbonaceous material incorporated therein in combination with a higher content of fine iron oxide particles having a particle size of 10 μm or less in the metal oxide (namely, iron ore) to increase the number of contacts between the iron oxide particles. According to this process, even if the carbonaceous material exhibits lower maximum fluidity in softening melting, the contact area between the iron oxide particles can be increased to enhance the thermal conductivity inside the agglomerates with the carbonaceous material incorporated therein. This results in a larger number of contacts between particles metallized by heating reduction so that the sintering thereof is promoted to provide high-strength reducing iron.
If, however, a reduced iron containing about 2% to 10% by mass of residual carbon is produced at about 10,000 lb/in2 (703 kg/cm2), a carbonaceous material having a high fixed carbon content must be generally used for increasing the content of elemental iron to ensure sufficient reduced iron strength. The above process for producing reduced iron therefore seems to require a high-grade bituminous coal having a high fixed carbon content and a volatile matter content of up to 35% by mass.
Such a high-grade bituminous coal, which has high quality with a high fixed carbon content, poses the problem of high cost due to small reserves and limited sources. On the other hand, coals having low fixed carbon contents, including subbituminous coal and other ranks of coals with lower degrees of coalification than subbituminous coal, are potential raw materials for steelmaking because of large reserves, unlimited sources, and low cost. If, however, subbituminous coal, which has a low fixed carbon content, or a coal with a lower degree of coalification, such as lignite, is used, the mixing ratio of the carbonaceous material to iron oxide, namely iron ore powder, must be increased; fixed carbon contributes greatly to the reduction of metal oxide such as iron oxide.
An increase in the content of coal with a low degree of coalification results in a relative decrease in the content of elemental iron in a green compact. This decreases bonding strength due to, for example, sintering by reduction, and thus decreases the strength of reduced iron. A reduced iron with decreased strength powders on impact when, for example, discharged from a rotary hearth furnace with a discharger. The powdered reduced iron, which has an increased specific surface area, is readily reoxidized by contact with oxidizing gases such as carbon dioxide and steam in the rotary hearth furnace. The resultant reduced iron is therefore less valuable as a semi-finished product, and exhibits poor handling properties because of its powdered form. Unfortunately, additionally, the powdered reduced iron, which has low bulk density, cannot be melted in a melting furnace because the powder floats over a slag layer.
On the other hand, a decreased content of carbonaceous material with a low fixed carbon content provides higher reduced iron strength. In this case, however, a metal oxide such as iron oxide cannot be sufficiently reduced because of the insufficient content of fixed carbon contributing to the reduction. If, for example, a reduced iron having a low residual carbon content is melted to produce hot metal, a carbonaceous material must be added to the hot metal to achieve the required carbon content. The addition of carbon to the hot metal increases the consumption of carbonaceous material because of its low yield, and may fail to achieve a target carbon concentration.
According to the process in which the proportion of fine iron oxide particles with a particle size of 10 μm or less is increased, the content of fine iron oxide particles with a particle size of 10 μm or less must be increased as the maximum fluidity of carbonaceous material is decreased. This process requires an additional step for providing finer particles. The use of coarse iron oxide particles with a particle size exceeding 10 μm alone cannot provide reduced iron with high strength.
The present invention focuses on the above problems in the related art. An object of the present invention is to provide agglomerates with a carbonaceous material incorporated therein that are prepared with high-VM coal, which is widely and abundantly produced and is less expensive, and that can provide high-strength reduced metal without the use of finer metal oxide particles, and also provide a process for producing reduced metal using the agglomerates.